Integrated monolithic circuits are complicated structures, in particular if they have a high scale of integration, and necessitate great development effort and extensive preliminary work before they can go into volume production. High piece numbers are therefore desired for each integrated circuit. one occasionally requires several versions of a certain integrated circuit which often differ only slightly. For example it should be possible to choose between an RC oscillator and a quartz oscillator as a clock generator.
A conventional method for providing several similar versions of a certain type of integrated circuit is to provide all components for all versions of this certain type of integrated circuit on one and the same chip and to apply different conduction patterns for the various versions. However different masks are needed for producing the particular conduction pattern required.
This effort can be avoided by equipping one and the same integrated circuit with all circuit parts and conduction patterns necessary for its various versions and equipping this integrated circuit with switches for internally switching to the particular desired version with the aid of corresponding switch control signals. This also leads to the advantage that such an integrated circuit can be switched from the initially selected version to another version during the development phase or even during use.
For this purpose one can use a storage circuit of the type described in Design+Elektronik, no. 13, 27th June 1989, Markt & Technik, pages 46, 48 and 49, and in Electronic Engineering, September 1988, pages 44, 45, 48, 50 and 54. The switches of the integrated circuit can thereby be controlled in accordance with the storage state of the storage circuit.
As is well-known, erasable programmable read only memory (EPROM) transistors have a floating gate between the conduction channel and the control gate. The floating gate of the EPROM transistor contains no charges in the unprogrammed state, while it contains charges in the programmed state. Looking at an n-channel state. The EPROM transistor therefore becomes conductive when a voltage of about 1.9 V is applied to the control gate. With the programmed n-channel EPROM transistor negative charges are found on the floating gate, which means that such a transistor only conducts in the programmed state as of a control gate voltage of more than 5 V.
This different behavior between the programmed and unprogrammed EPROM transistor is utilized for information storage. This information can be used for example to control in the above-mentioned way the switching state of hardware switches located on the same semiconductor chip as the storage circuit.
With a storage circuit of the above-mentioned type no error can occur in terms of false storage information being delivered. After disturbances this storage circuit always independently resumes the correct circuit state in which it reliably delivers the programmed storage information. If there is a supply voltage drop, whether due to disturbances or to a deliberate shut-off, it suffices merely to turn the supply voltage back on to deliver the correct storage information again. No readout signal is required.
Such a storage circuit is not only suitable for providing switch control information for switches, but it can also be used advantageously wherever information is to be reliably stored and correctly outputted, over a very long time and without disturbances. This storage circuit is also excellently suited for nonvolatile memories for example.
It is generally necessary to perform repeated testing on the storage circuit and the circuit controlled by it. It may also become necessary to subsequently change the functions of the circuit controlled by the storage circuit, which means changing the storage fill of the storage circuit and thus reprogramming it. Testing and/or reprogramming might be necessary during operation of the controlled circuit without this operation being interrupted or impaired.